SVN is supported by Emacs' built-in version control system.
I don't think there's anything comparable to magit for SVN, partly because git is much more capable than SVN, and partly because fewer and fewer people have been using SVN since git came along.
The Foundation I work for also uses Subversion exclusively, since quite a few features they need would not be handled as easily using git (some of them: huge monorepos, centralised workflow, excellent TortoiseSvn client).
Some of the engineers use dsvn. Some use psvn. I personally use the built in Emacs VC system.
See also Subversion on EmacsWiki.